The advent of hybrid scanners, combining complementary modalities, has revolutionized imaging; enhancing clinical practice and biomedical research. The standard paradigm is to combine an anatomical imaging method (X-ray CT, for example) with a functional method (PET, for example). In this project we propose a shift of this paradigm by investigating the melding of two complementary, functional imaging methods. Specifically, we plan to integrate a PET scanner with an electron paramagnetic resonance imaging (EPRI) scanner. EPRI is a relatively new method, capable of mapping the in vivo chemical characteristics of tissue. A combined PET- EPRI scanner has the promise to provide new insights into physiologic interactions in microenvironments not currently attainable with current imagers. Development of the PET/EPRI system is technically challenging, requiring unique approaches to scanner design, construction and testing. We will utilize a novel PET scanner that replaces a ring of discrete detector modules with a solid annulus of scintillator (spatial resolution= ~1mm). This design eliminates conductive material needed to construct discrete detector modules. Use of annular scintillator results in high detection sensitivity (~10%) due to elimination of gaps between discrete modules. It also permits estimation of event depth-of-interactions in the detector by correlating light cone shape with depth, a capability not possible with discrete detectors. Arrays of silicon photomultipliers (SiPM), which are not affected by the presence of magnetic fields present in EPR systems, will be used to detect scintillator light. The EPRI system will utilize the rapid scanning method to produce spatial maps of spectra obtained from molecular probes (resolution <1mm). We plan to use a nested design in which the animal handling enclosure, where the system?s RF resonator, shielding, EPR scan coils, PET scanner and gradient coils are combined into a single, compact PET-EPRI insert. The insert will be mounted on a computer-controlled gantry, permitting positioning inside the electro-dipole magnet (400G) required for EPR. The portable animal enclosure will include a grid of fiducial markers to facilitate registration with images acquired on our group?s small animal, MRI scanner. Initial testing of the PET/EPRI scanner will be performed using standardized testing protocols and phantoms emulating physiologic microenvironments. To demonstrate the potential utility of the new system, it will be used in a study of the interaction between the intra-and extracellular components of tumor microenvironments. This investigation will utilize MMTV-PyMT-transgenic mice that spontaneously-develop breast cancer. 18F-fluorodeoxyglucose (FDG)-PET imaging will be used to quantify areas of enhanced glycolysis (intracellular component), while EPR imaging, using a multifunctional trityl probe, will be used to quantify hypoxic and acidic areas (extracellular component) at extended time points as hyperplasia evolves in to metastatic breast cancer. The resulting information will help explore the hypothesis that carcinogenic progression is an evolutionary process, and illustrate the unique capabilities of a combined PET-EPRI scanner.